Alzheimer's disease (AD) is a progressive neurodegenerative disease resulting in senile dementia and afflicts four million people in the United States alone (see generally Sloe, TINS, 16:403-409 (1993); Hardy et al., WO 92/13069; Sloe, J. Neuropathol. Exp. Neurol., 53:438-447 (1994); Duff et al., Nature, 373:476-477 (1995); Games et al., Nature, 373:523 (1995)). Broadly speaking, the disease falls into two categories: late onset, which occurs in old age (65+ years); and early onset, which develops well before the senile period, i.e., between 35 and 60 years. In both types of disease, the pathology is the same but the abnormalities tend to be more severe and widespread in cases beginning at an earlier age. The disease is characterized by at least two types of lesions in the brain, senile plaques and neurofibrillary tangles. Neurofibrillary tangles are intracellular deposits of microtubule-associated tau protein consisting of two filaments twisted about each other in pairs. Senile plaques are areas of disorganized neuropil up to 150 microns across (visible by microscopic analysis of sections of brain tissue) and have extracellular amyloid deposits at the center. A principal component of such plaques is β-amyloid peptide (Aβ) (see Forsyth Phys. Ther., 78:1325-1331 (1998)). Additional proteins found in the plaques include laminin as described by Murtomaki et al., J. Neurosci. Res., 32:261-273 (1992), apoE, acetylcholinesterase, and heparin sulfate proteoglycans, as described by Yan et al., Biochim. Biophys. Acta, 1502:145-57 (2000).
Amyloid precursor protein (APP) is a synaptic single-pass transmembrane protein that is best known for its involvement in AD. In AD patients, amyloid plaques containing aggregated Aβ peptide appear in specific brain regions, triggering an inflammatory response, neuronal cell death, and gradual cognitive decline. One mechanism by which Aβ is derived from APP is cleavage of APP at an extracellular position (β site), followed by an unusual cleavage within the APP transmembrane segment (γ site), producing a fragment of 39-43 amino acids of APP.
Several mutations within the APP protein have been correlated with the presence of Alzheimer's disease (Goate et al., Nature, 349:704-06 (1991) (valine717 to isoleucine); Harlin et al., Nature, 353:844-46 (1991) (valine717 to glycine); Murrell et al., Science, 254:97-99 (1991) (valine717 to phenylalanine); Mullan et al., Nature Genet., 1:345-47 (1992) (a double mutation changing lysine595methionine596 to asparagine595leucine596)). Such mutations are thought to cause Alzheimer's disease by increased or altered processing of APP to Aβ, particularly processing of APP to increased amounts of the long form of Aβ (i.e., Al -42 and A1-43). Mutations in other genes, such as the presenilin genes PS1 and PS2, are thought to indirectly affect processing of APP to generate increased amounts of long form Aβ (Hardy, TINS, 20:154 (1997)). These observations indicate that Aβ, and particularly its long form, is a causative element in Alzheimer's disease (Velez-Pardo et al., Gen. Pharm., 31(5):675-81 (1998)).
The Aβ peptide has also been implicated in neuropathological defects seen in individuals inflicted with Down's syndrome. For example, almost all individuals with Down's syndrome, who have an extra copy of chromosome 21, show neuropathological changes similar to those seen in Alzheimer's disease, if they survive into their 40s. This has been attributed to excess production of β-amyloid protein, which is encoded by the APP gene on chromosome 21.
Several proteins have been investigated for possible interactions with Aβ. These include the receptor for advanced glycation endproducts, RAGE (see Yan et al., Nature, 382:685-91 (1996)), the scavenger receptor (Khoury et al., Nature, 382:716-719 (1996); and Paresce et al., Neuron 17:553-65 (1996)), the endoplasmic reticulum-associated amyloid-beta biding protein (ERAB) (Yan et al., Nature, 389:689-695 (1997)), α4 or α7 nicotinic acetylcholine receptor (Wang et al., J. Neurochem., 75:1155-1161 (2000) and Wang et al., J. Biol. Chem., 275:5626-5632 (2000)), and the low affinity p75 NGF receptor (see Yaar et al., J. Clin. Invest., 100:2333-2340 (1997)). Additionally, Aβ has been reported to mediate adhesion of cells in a β1-integrin subunit-dependent manner when coated onto plates. (Ghiso et al., Biochem. J., 288:1053-59 (1992); Matter et al., J. Cell Bio., 141:1019-1030 (1998)). However, the mechanism(s) by which Aβ may mediate neurodegeneration remains unclear. The existence and nature of other cellular proteins that may have roles in the process is also largely unclear.
Various signaling pathways have been implicated in the pathogenesis of AD, including, p38, erk and c-jun N-terminal kinases (JNK) kinase cascades. For example, chronic stimulation of the JNKs has been shown to cause neuronal cell death in several disease paradigms, including in vitro AD models. Additionally, it has been shown that expression of kinase-deficient JNK3 protects against Aβ-induced cell death. However, it is not entirely clear which of these pathways and/or the signaling molecules are the key players in the development of the disease. Accordingly, elucidation of the signaling pathways as well as the intermediates of those pathways will help in understanding the pathology of AD and in developing therapeutic strategies to combat AD.